The present invention relates to arc lamps and, more particularly, to a method of shaping the light pulses emitted by an arc lamp.
Pulsed arc lamps have many applications in warfare, in medicine and in the fabrication of semiconductor devices.
FIG. 1 shows a prior art circuit 10 for driving an arc lamp 12. Circuit 10 includes a DC power supply 14, a capacitor 16, a gating switch 18, a diode 20 and a coil 22 connected as shown. In the circuit as drawn in FIG. 1, electrical current from circuit 10 is fed to the anode 24 of arc lamp 12, and the cathode 26 of arc lamp 12 is grounded. Alternatively, electrical current from circuit 10 is fed to cathode 26 and anode 24 is grounded.
Power supply 14 supplies electrical current at a voltage of between 200V and 400V. Capacitor 16 is relatively large, to act as an energy reservoir. In the example shown, capacitor 16 has a capacitance of two millifarads. Gating switch 18 is shown as an insulated gate bipolar transistor (IGBT). Gating switch 18 is opened and closed by a driver 28 to provide pulses of electrical current from power supply 14 to arc lamp 12. Diode 20 serves to discharge coil 22 when gating switch 18 is opened. Coil 22 has a ferrite core and is used to shape the voltage pulses from gating switch 18. Coil 22 also is the secondary coil of a transformer 30 whose primary coil 32 is energized by a trigger pulse source (igniter) 34.
To turn on arc lamp 12, igniter 34 is turned on to create an ignition pulse that provides a high (˜20KV) voltage, low current trigger pulse between anode 24 and cathode 26 to create a conductive path from anode 24 to cathode 26 by ionizing the gas, that fills arc lamp 12, between anode 24 and cathode 26. Then an operating voltage pulse at a lower voltage of between 200V and 400V is introduced to arc lamp 12 by closing and then opening gating switch 18. FIG. 2 shows the shapes of the voltage VS provided by gating switch 18 and the resulting electrical current IL in arc lamp 12 as a function of time t. VS is a square voltage pulse that lasts from time t1, when gating switch 18 is closed, to time t2, when gating switch 18 is opened. While gating switch 18 is closed, IL is (VS/L)(t−t1), where L is the inductance of coil 22. Initially, the ferrite core of coil 22 gives coil 22 a high inductance L, so the slope of IL(t) is very low. When the ferrite core of coil 22 becomes saturated, at time ts, the inductance L of coil 22 falls to the inductance of an air coil, and the slope of IL(t) increases. IL(t) rises to a maximum value of ILmax at time t2. When gating switch 18 is opened at time t2, IL(t) starts to decay exponentially with a time constant of L/R where R is the effective resistance of diode 20, arc lamp 12, coil 22 and the wires that connect them. The overall shape of the current pulse IL that actually flows through arc lamp 12 is approximately triangular. The intensity of the light emitted by arc lamp 12 is proportional to IL.
Some applications of pulsed arc lamps require that the shape of the intensity profile of the light pulses be other than triangular, for example square. Perkin-Elmer of Wellesley Mass., USA, has developed a rather complicated circuit for driving an arc lamp in a way that provides light pulses with square intensity profiles. This circuit is described on the World Wide Web at http://optoelectronics.perkinelmer.com/content/RelatedLinks/pulsed_power_applications.pdf
This circuit is considerably more complicated than prior art circuit 10.